1. Field of the Invention
The present invention relates to novel block copolymers of polyalkylenes and halogenated polystyrenes, and more particularly to compositions retaining advantageous physical properties but having reduced flammability.
1. Description of the Prior Art
Saturated polyalkylenes, such as polyethylene, polypropylene, etc., display elastomeric properties which make them useful in varied applications. However, polyalkylenes have a higher flammability than is desired for certain applications. It is therefore advantageous to prepare a polymer which retains the elastomeric properties associated with the polyalkylenes, but which has reduced flammability.
As reported in U.S. Pat. No. 3,576,912, issued to Winkler on Apr. 27, 1971, halogenation of unsaturated polymers has been carried out in the prior art for preparation of fire resistant polymers. It has been found, however, that halogenation to an extent effective in providing the desired degree of fire resistance results in a loss of the elastomeric properties of the unhalogenated polymer. The Winkler patent further states that halogenation of butyl rubber has been somewhat successful, and that this results from halogenation at the conjugated diene linkages, rather than at saturated bonds.
The Winkler patent similarly discusses the preparation of block copolymers of monovinyl arenes and conjugated dienes in which the diene is selectively halogenated. The Winkler patent describes the substantially exclusive halogenation and/or quaternization of the conjugated diene polymer block (such as polybutadiene or polyisoprene), with the thermoplastic terminal blocks being substantially unaffected. The Winkler patent involves only selected classes of halogenating agents, namely N-haloimides, bromoanilides, bromoamines, and N-monohalohydantoins, or the corresponding chlorinated halogenating agents. The Winkler process intentionally provides for essentially no reaction in the monovinyl arene polymer blocks for the express purpose of maintaining the original physical properties of the block copolymer.
Related compositions are disclosed in U.S. Pat. No. 3,607,977, issued to Taylor, Cotton and Winkler on Sept. 21, 1971. The latter patent describes the preparation of block copolymer compositions having substantially increased polar character to promote adhesion to polar surfaces, as well as reduced flammability. The compositions comprise blends of two different block copolymers. The first block copolymer has the general configuration of A--B--(B--A).sub.1-5, wherein the "A" blocks are monovinyl arene blocks and the "B" blocks are conjugated diene blocks. The first block copolymer thus may comprise, for example, polystyrene-polybutadiene-polystyrene. The second block copolymer has the general configuration of A--C--(C--A).sub.1-5, in which the "A" blocks are monovinyl arene blocks and the "C" blocks may be polar, including halogenated, derivatives of the "B" blocks. Again, the halogenation is provided in the conjugated diene blocks and not the monovinyl arene blocks. The halogenation of the diene blocks is described, for example, as by the addition of HBr across the diene unsaturated linkages.
There has remained a desire for polyalkylene polymers which retain advantageous elastomeric properties, but which have reduced flammability. The present invention provides halogenated styrenic block copolymers which satisfy these criteria.